Flux for soldering and circuit board

ABSTRACT

A flux contains resin having film forming ability, activator, solvent, and at least one complex selected from silver complex and copper complex. The flux is used when soldering is performed onto a circuit having electroless nickel plating or further having gold plating on the electroless nickel plating. Allowing a barrier layer of silver or copper to deposit on the surfaces of lands suppresses the diffusion of nickel into the melted solder alloy during soldering, and also prevents phosphorous concentration. This improves the bonding strength of soldering and suppresses the reduction deposition of silver and/or copper to portions other than circuit patterns.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flux used when soldering variouselectronic components onto a circuit board, and more particularly to aflux used when soldering is conducted onto copper lands havingelectroless nickel plating or further having gold plating on theelectroless nickel plating, as well as a circuit board.

2. Description of Related Art

Traditionally, when electronic components are generally soldered onto aprinted circuit board, soldering to copper lands formed on the printedcircuit board has been done by using tin/lead alloy solder and lead-freesolder. Generally, to prevent copper oxidation, electroless nickelplating to the surfaces of the lands is conducted in advance, and goldplating to the surface is further conducted in advance by using a nickelplating layer as a barrier layer.

However, since hypophosphite is used as reducing agent in theelectroless nickel plating to the above-mentioned lands, a tracequantity of phosphorous is contained in a nickel plating coat.Therefore, when soldering with the use of solder alloy is conducted tothe surface of the electroless nickel plating or the surface having thegold plating on the electroless nickel plating, gold and the nickel inthe nickel plating diffuse into the melted solder alloy. Then, at theboundaries between the nickel plating layer and the solder alloy,phosphorus segregates locally thereby to produce portions at whichphosphorous is extremely concentrated, and in some cases bondingstrength is lowered to strip soldering.

As a method for preventing such nickel segregation, the presentapplicant previously proposed a method of adding various metallic saltsinto flux (Japanese Patent Laid-Open No. 2003-236695). Specifically,inclusion of metallic salt in flux permits the metal in the metallicsalt to be substituted by nickel and deposited. This suppresses thereaction of nickel with the metal in solder alloy, thereby retarding thediffusion of nickel in the surfaces of the lands into the solder alloy.

Unfortunately, when the salts of silver and copper are used as additivesto flux, silver or copper is liable to liberate and depositindependently. Therefore, for example, in a soldering method includingthe steps of printing flux overall onto a substrate having lands, andplacing solder balls on the lands and then having the solder ballsreflow so as to connect them to the lands, metal deposits from theprinted flux and forms a thin metallic film between the lands. Since themetallic film cannot be removed by cleaning, there arises the problem ofimpairing electrical insulating properties between the lands.

SUMMARY OF THE INVENTION

One advantage of the present invention is to improve the bondingstrength of soldering by suppressing the diffusion of nickel into themelted solder alloy during soldering, and to suppress the reductiondeposition of silver and/or copper to portions other than a circuitpattern.

A flux of the present invention is particularly used when soldering isperformed onto a circuit having electroless nickel plating or furtherhaving gold plating on the electroless nickel plating, and containsresin having film forming ability, activator, solvent, and at least onecomplex selected from silver complex and copper complex. In the presentinvention, the term “complex” means silver or copper compound having atleast one coordinate bond.

The complex in the present invention has no possibility that whenheating this, silver or copper independently causes reductiondeposition. Instead, the complex deposits due to the substitution bymetal that is large in ionization tendency, namely nickel. Hence, it isable to form a barrier layer of silver and/or copper only on thesurfaces of the lands, without possibility of forming a thin metallicfilm between conductor patterns of a substrate, and impairing electricalinsulating properties between the lands. The barrier layer of silverand/or copper so formed on the surfaces of the lands suppresses thediffusion of nickel into solder alloy, and prevents the formation of aphosphorous concentrated layer, thereby to improve the bonding strengthof the solder.

In other words, a circuit board of the present invention includes landshaving on their surfaces electroless nickel plating or further havinggold plating on the electroless nickel plating, and a barrier layer ofat least one selected from silver and copper to be formed on thesurfaces of the lands.

The present invention is also effective in the surface further havinggold plating on electroless nickel plating, not only the case of adirect soldering to an electroless nickel plating layer. Specifically,although during soldering, gold plating coat diffuses momentarily intosolder alloy, and nickel and solder alloy make direct contact to formthe above-mentioned phosphorous concentrated layer, the presentinvention can prevent this effectively.

In the present invention, silver or copper is substituted by nickel anddeposited due to heating at the time of soldering, thus enabling to forma barrier layer on the surfaces of the lands. Therefore, preferably, thecomplex of the present invention does not cause substitution depositionof silver and copper at temperatures of less than 150° C., and initiatessubstitution deposition when the temperature is elevated to not lessthan 150° C. during soldering. Further, the complex preferably does notcause metal deposition at portions other than the circuit pattern attemperatures of not more than 280° C., at which it might be exposedduring soldering.

A first soldering method of the present invention includes the step ofprinting or applying the aforesaid flux for soldering onto a substratehaving copper lands which have on their surfaces electroless nickelplating or further have gold plating on the electroless nickel plating,and then heating to form a barrier layer of at least one selected fromsilver and copper on the surfaces of the lands; and the step of placingsolder balls on the lands with the barrier layer formed on the surfaces,and then heating to have the solder balls reflow so as to connect themto the lands. From the point of view of environmental impact, solderballs to be used are preferably lead-free.

Thus, prior to soldering, the flux of the present invention is appliedand heated to form a barrier layer on the surfaces of the lands.Thereby, the barrier layer protects the surfaces of the lands, and inthe succeeding soldering step, the diffusion of nickel can be preventedto improve bonding strength. In this case, the flux to be used in thesoldering step may not be the flux of the present invention.

A second soldering method of the present invention includes the step ofprinting or applying the aforesaid flux for soldering on a substratehaving copper lands which have on their surfaces electroless nickelplating or further have gold plating on the electroless nickel plating;and the step of placing solder balls on the lands and heating to havethe solder balls reflow so as to form a barrier layer of at least oneselected from silver and copper on the surfaces of the lands, andconnect the solder balls via the barrier layer to the lands.

A third soldering method of the present invention includes printingpaste that is a mixture of the aforesaid flux for soldering and solderpowder, on a substrate having lands which have on their surfaceselectroless nickel plating, and then heating to have the paste reflow soas to form solder alloy on the lands.

The present invention also intends to provide a circuit board in whichsolder is connected by the soldering methods as above described.

Other objects and advantages of the present invention will be apparentfrom the following detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flux of the present invention contains resin having film formingability, activator, solvent, and at least one complex selected fromsilver complex and copper complex.

As resin having film forming ability, for example, rosin orthermoplastic acrylic can be used. Acrylic resin has a molecular weightof not more than 10000, preferably 3000 to 8000. When the molecularweight exceeds 10000, cracking resistance and stripping resistance maydrop. In order to aid active action, acid value is preferably not lessthan 50. Softening point is preferably not more than 230° C., because itis necessary that acrylic resin is softened during soldering.

Therefore, examples of suitable acrylic resin are composed from monomershaving polymerizing unsaturated group, such as (metha)acrylic acid andester thereof (e.g., methyl(metha)acrylate, etc.), crotonic acid,itaconic acid, maleic acid(maleic anhydride) and ester thereof,(metha)acrylonitrile, (metha)acrylamide, vinyl chloride, and vinylacetate. These are preferably polymerized with the use of catalyst, suchas peroxide, by radical polymerization such as bulk polymerizationmethod, solution polymerization method, suspension polymerizationmethod, and emulsion polymerization method.

As rosin, it is able to use rosins and derivatives thereof, which havetraditionally been used in fluxes. As rosin and derivative thereof,general gum, tall, and wood rosin are usable. Examples of thederivatives thereof are heat treated resin, polymerized rosin,hydrogenated rosin, formylated rosin, rosin ester, rosin modified maleicresin, rosin modified phenol resin, and rosin modified alkyd resin.

The content of resin having film forming ability is in an amount of 20to 80% by weight, preferably, 30 to 65% by weight of the total amount offlux. When the content is less than 20% by weight, wettability maydeteriorate. On the other hand, when the content is over 80% by weight,viscosity control is impossible and hence operability may deteriorate.

The activator is not particularly limited, for example, amine salt ofchlorohydric acid or hydrobromic acid, carboxylic acid or amine saltthereof, etc. Specifically, chlorohydric acid salt or hydrobromic acidsalt such as methyl amine, dimethyl amine, trimethyl amine, ethyl amine,diethyl amine, triethyl amine, n-propyl amine, di-n-propyl amine,tri-n-propyl amine, isopropyl amine, diisopropyl amine, triisopropylamine, butylamine, dibutylamine, monoethanol amine, diethanol amine andtriethanol amine, etc.; or organic acids such as oxalic acid, malonicacid, succinic acid, adipic acid, glutaric acid, diethyl glutaric acid,pimelic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid,lactic acid, diglycolic acid, capric acid, lauric acid, myristic acid,palmitic acid, linoleic acid, oleic acid, benzoic acid, hydoroxypivalicacid, dimethylolporopionic acid, citric acid, malic acid, glyceric acid,stearic acid, arachic acid, behenic acid and linoleic acid, etc. oramine salt thereof. Alternatively, rosin containing rosin acid issuitably usable as activator.

While the content of activator is not particularly limited, it ispreferably in an amount of 0.1 to 30% by weight of the total amount offlux. When the content is less than 0.1% by weight, the function ofactivator, namely, the active power for removing and cleaning metallicoxide on the metal surface is insufficient, so that solderability may belowered. On the other hand, when the content is over 30% by weight, thefilm forming ability of flux decreases and hydrophilicity increases, sothat corrosivity and insulation performance may be lowered. In the caseof using rosin so as to perform both functions of resin and activator,the contents of rosin and derivative thereof must be in an amount,within which both functions of these are not impaired.

As solvent, there are, for example, alcohol solvents such as ethylalcohol, isopropyl alcohol, ethyl cellosolve, butyl carbitol and hexylcarbitol (diethylene glycol monohexyl ether); and ester solvents such asethyl acetate and butyl acetate; and hydrocarbon solvents such astoluene and turpentine oil.

Solvent is preferably added in an amount of 5 to 70% by weight of thetotal amount of flux. When the amount of addition of solvent is lessthan 5% by weight, the viscosity of flux increases, and theapplicability of the flux might deteriorate. On the other hand, when theamount of addition of solvent is over 70% by weight, the ratio ofeffective compositions (e.g., resin) as flux is reduced, so thatsolderability may be lowered.

A complex in the present invention is preferably one which does notsubstantially cause substitution deposition of silver and/or copper on acircuit pattern at temperatures of less than 150° C., and causessubstitution deposition at temperature of not less than 150° C., anddoes not substantially cause deposition of silver and/or copper atportions other than the circuit pattern at temperatures of not more than280° C.

As a complex satisfying such temperature conditions of substitutiondeposition, there are, for example, complexes of silver ion and/orcopper ion and phosphine, nitrogenated heterocyclic compound, or acompound having thiol, thiother or disulfide bond.

As the phosphines, at least one selected from aryl phosphines and alkylphosphines represented by the following general formula 1. Thephosphines can be used alone, or a mixture of two or more of them can beused.

wherein R₁, R₂ and R₃ each represents a substituted or non-substitutedaryl group, or a substituted or non-substituted chain or cyclic alkylgroup having 1 to 8 carbon atoms; hydrogen of the aryl group may besubstituted with an alkyl having 1 to 8 carbon atoms, an alkoxy grouphaving 1 to 8 carbon atoms, a hydroxyl group, an amino group or ahalogen atom at any position; hydrogen of the alkyl group may besubstituted with an alkoxy group having 1 to 8 carbon atoms, an arylgroup, a hydroxyl group, an amino group or a halogen at any position;and R₁, R₂ and R₃ may be the same or different.

As the aryl phosphines, for example, such as triphenyl phosphine,tri(o-, m- or p-tolyl) phosphine and tri(p-methoxyphenyl)phosphine aresuitable. Since as the alkyl phosphines, for example, such as tributylphosphine, trioctyl phosphine, tris(3-hydroxypropyl)phosphine andtribenzyl phosphine are preferably used.

Among these compounds, triphenyl phosphine, tri(p-tolyl)phosphine,tri(p-methoxyphenyl)phosphine, trioctyl phosphine andtris(3-hydroxypropyl)phosphine are used particularly preferably, andtriphenyl phosphine, tri(p-tolyl)phosphine andtri(p-methoxyphenyl)phosphine are used most preferably.

As the nitrogenated heterocyclic compounds, for example, at least oneselected from five-ring compound, six-ring compound and derivativesthereof.

As the nitrogenated five-ring compound, for example, the azoles such astetrazole, triazole, benzotriazole, imidazole, benzimidazole, pyrazole,indazole, thiazole, benzothiazole, oxazole, benzoxazole, pyrrole, indoleand derivatives thereof can be used alone, or a mixture of two or moreof them can be used.

Examples of the tetrazole and derivative thereof include tetrazole,5-aminotetrazole, 5-mercapto-1-methyltetrazole and5-mercapto-1-phenyltetrazole. Examples of the triazole, benzotriazoleand derivative include 1,2,3-triazole, 1,2,3-triazole-4,5-dicarboxylicacid, 1,2,4-triazole, 3-amino-1,2,4-triazole, 4-amino-1,2,4-triazole,3-mercapto-1,2,4-triazole, benzotriazole, 5-methyltriazole,tolyltriazole, benzotriazole-5-carboxylic acid, carboxybenzotriazole,4-aminobenzotriazole, 5-aminobenzotriazole, 4-nitrobenzotriazole,5-nitrobenzotriazole and 5-chlorobenzotriazole.

Examples of the imidazole, benzimidazole and derivative thereof includeimidazole, 1-methylimidazole, 1-phenylimidazole, 2-methylimidazole,2-ethylimidazole, 2-propylimidazole, 2-butylimidazole,2-phenylimidazole, 4-methylimidazole, 4-phenylimidazole,2-aminoimidazole, 2-mercaptoimidazole, imidazole-4-carboxylic acid,benzimidazole, 1-methylbenzimidazole, 2-methylbenzimidazole,2-ethylbenzimidazole, 2-butylbenzimidazole, 2-octylbenzimidazole,2-phenylbenzimidazole, 2-trifluoromethylbenzimidazole,4-methylbenzimidazole, 2-chlorobenzimidazole, 2-hydroxybenzimidazole,2-aminobenzimidazole, 2-mercaptobenzimidazole,2-methylthiobenzimidazole, 5-nitrobenzimidazole andbenzimidazole-5-carboxylic acid.

Examples of the pyrazole, indazole and derivative thereof includepyrazole, 3-methylpyrazole, 4-methylpyrazole, 3,5-dimethylpyrazole,3-trifluoromethylpyrazole, 3-aminopyrazole, pyrazole-4-carboxylic acid,4-bromopyrazole, 4-iodopyrazole, indazole, 5-aminoindazole,6-aminoindazole, 5-nitroindazole and 6-nitroindazole.

Examples of the thiazole, benzothiazole and derivative thereof includethiazole, 4-methylthiazole, 5-methylthiazole, 4,5-dimethylthiazole,2,4,5-trimethylthiazole, 2-bromothiazole, 2-aminothiazole,benzothiazole, 2-methylbenzothiazole, 2,5-dimethylbenzothiazole,2-phenylbenzothiazole, 2-chlorobenzothiazole, 2-hydroxybenzothiazole,2-aminobenzothiazole, 2-mercaptobenzothiazole and2-methylthiobenzothiazole.

Examples of the oxazole, benzoxazole and derivative thereof includeisoxazole, anthranyl, benzoxazole, 2-methylbenzoxazole,2-phenylbenzoxazole, 2-chlorobenzoxazole, 2-benzoxazolinone and2-mercaptobenzoxazole.

Examples of the pyrrole, indole and derivative thereof include pyrrole,2-ethylpyrrole, 2,4-dimethylpyrrole, 2,5-dimethylpyrrole,pyrrole-2-carboxyaldehyde, pyrrole-2-carboxylic acid,4,5,6,7-tetrahydroindole, indole, 2-methylindole, 3-methylindole,4-methylindole, 5-methylindole, 6-methylindole, 7-methylindole,2,3-dimethylindole, 2,5-dimethylindole, 2-phenylindole, 5-fluoroindole,4-chloroindole, 5-chloroindole, 6-chloroindole, 5-bromoindole,4-hydroxyindole, 5-hydroxyindole, 4-methoxyindole, 5-methoxyindole,5-aminoindole, 4-nitroindole, 5-nitroindole, indole-3-carboxyaldehyde,indole-2-carboxylic acid, indole-4-carboxylic acid, indole-5-carboxylicacid, indole-3-acetic acid, 3-cyanoindole 5-cyanoindole and carbazole.

As the nitrogenated six-ring compound, for example, pyridine,pyridazine, pyrimidine, pyrazine, triazine, quinoline or phenanthrolineand derivatives thereof can be used alone, or a mixture of two or moreof them can be used.

Among these compounds, pyridine, 2,2′-bipyridyl, nicotinic acid,pyridazine, pyrimidine, uracil, pyrazine, 1,3,5-triazine, cyanuric acid,quinoline, 8-hydroxyquinoline, isoquinoline and 1,10-phenanthroline arepreferably used.

Furthermore, among these compounds, tetrazole,5-mercapto-1-phenyltetrazole, 1,2,3-triazole, 1,2,4-triazole,3-mercapto-1,2,4-triazole, benzotriazole, tolyltriazole,carboxybenzotriazole, imidazole, 2-mercaptoimidazole, benzimidazole,2-octylbenzimidazole, 2-phenylbenzimidazole, 2-mercaptobenzimidazole,2-methylthiobenzimidazole, pyrazole, indazole, thiazole, benzothiazole,2-phenylbenzothiazole, 2-mercaptobenzothiazole,2-methylthiobenzothiazole, isoxazole, anthranil, benzoxazole,2-phenylbenzoxazole, 2-mercaptobenzoxazole, pyrrole,4,5,6,7-tetrahydroindole, indole, pyridine, 2,2′-bipyridyl, nicotinicacid, pyridazine, pyrimidine, uracil, pyrazine, 1,3,5-triazine, cyanuricacid, quinoline, 8-hydroxyquinoline, isoquinoline and1,10-phenanthroline are preferred.

Furthermore, among these compounds, 5-mercapto-1-phenyltetrazole,3-mercapto-1,2,4-triazole, benzotriazole, tolyltriazole,carboxybenzotriazole, imidazole, benzimidazole, 2-octylbenzimidazole,2-mercaptobenzimidazole, benzothiazole, 2-mercaptobenzothiazole,benzoxazole, 2-mercaptobenzoxazole, pyridine, 2,2′-bipyridyl,8-hydroxyquinoline and 1,10-phenanthroline are particularly preferred.

As the compound having thiol, thiother or disulfide bond, for example,methanethiol, ethanethiol, 1-propanethiol, 1-butanethiol,3-methyl-1-butanethiol, 2-propene-1-thiol, ethanedithiol,2-mercaptoethanol, 2,3-dimercapto-1-propanol, 2-aminoehanethiol,benzenethiol, trienethiol, 1,4-benzenethiol, trienedithiol,aminobenzenethiol, phenylmethanethiol, mercaptoacetic acid,2-mercaptopropionic acid, mercaptosuccinic acid, Lcysteine, methylsulfide, ethyl sulfide, dibutyl sulfide, divinyl sulfide, diphenylsulfide, dibenzyl sulfide, dimethyl disulfide, diethyl disulfide,methylpropyle disulfide and dithioglycolic acid can be used alone, or amixture of two or more of them can be used.

When the complex with the aforesaid silver ion and/or copper ion in thepresent invention is cationic, counter anion is required. As thiscounter anion, organic sulfuric acid ion, organic carboxylic acid ion,halogen ion, nitric acid ion or sulfuric acid ion are suitable, andorganic sulfuric acid ion is particularly preferred.

As the organic sulfonic acid used as the counter anions, at least oneselected from organic sulfonic acids represented by the followinggeneral formulas 2, 3 and 4 are preferably used.(X₁)_(n)—R₄—SO₃H   general formula 2:wherein R₄ represents an alkyl group having 1 to 18 carbon atoms, analkenyl group having 2 to 18 carbon atoms or an alkynyl group having 2to 18 carbon atoms, X₁ represents a hydrogen, a hydroxyl group, an alkylgroup having 1 to 8 carbon atoms or an alkoxy group, an aryl group, anaralkyl group, a carboxyl group or a sulfo group, n represents aninteger of 0 to 3, and X₁ may be bonded at any position of R₄.(X₂)_(n)—R₅—(SO₃H)_(m)   general formula 3:wherein R₅ represents an alkyl group having 1 to 18 carbon atoms or analkylene group having 1 to 3 carbon atoms, and when an alkylene group, ahydroxyl group may be bonded at any position of the alkylene group; X₂represents chlorine and/or fluorine; n represents an integer of not lessthan 1 and not more than the number of hydrogen capable of bonding withR₅; and m represents an integer of 1 to 3. X₂ may be bonded with R₅ atany position.

wherein X₃ represents a hydroxyl group, an alkyl group having 1 to 18carbon atoms, an alkoxy group having 1 to 8 carbon atoms, an aryl group,an aralkyl group, an aldehyde group, a carboxyl group, a nitro group, amercapto group, a sulfo group or an amino group, or two adjacent X₃(s)may form a ring to form a naphthalene ring with a benzene ring, and nrepresents an integer of 0 to 3.

Specific examples of preferred organic sulfonic acid includemethanesulfonic acid, methanedisulfonic acid, methanetrisulfonic acid,trifluoromethanesulfonic acid, ethanesulfonic acid, propanesulfonicacid, 2-propanesulfonic acid, butanesulfonic acid, 2-butanesulfonicacid, pentanesulfonic acid, hexanesulfonic acid, decanesulfonic acid,dodecanesulfonic acid, hexadecanesulfonic acid, octadecanesulfonic acid,2-hydroxyethanesulfonic acid, 1-hydroxypropane-2-sulfonic acid,3-hydroxypropane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid,2-hydroxybutanesulfonic acid, 2-hydroxypentanesulfonic acid,2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecanesulfonic acid,2-hydroxydodecanesulfonic acid, 1-carboxyethanesulfonic acid,2-carboxyethanesulfonic acid, 1,3-propanedisulfonic acid, allylsulfonicacid, 2-sulfoacetic acid, 2- or 3-sulfopropionic acid, sulfosuccinicacid, sulfomaleic acid, sulfofumaric acid, monochloromethanesulfonicacid, trichloromethanesulfonic acid, perchloroethanesulfonic acid,trichlorodifluoropropanesulfonic acid, perfluoroethanesulfonic acid,monochlorodifluoromethanesulfonic acid, trifluoromethanesulfonic acid,trifluoroethanesulfonic acid, tetrachloropropanesulfonic acid,trichlorodifluoroethanesulfonic acid, monochloroethanolsulfonic acid,dichloropropanolsulfonic acid, monochlorodifluorohydroxypropanesulfonicacid, benzenesulfonic acid, toluenesulfonic acid, xylenesulfonic acid,nitrobenzenesulfonic acid, sulfobenzoic acid, sulfosalicylic acid,benzaldehydesulfonic acid, phenolsulfonic acid, phenol-2,4-disulfonicacid, anisolesulfonic acid, 2-sulfoacetic acid, 2-sulfopropionic acid,3-sulfopropionic acid, sulfosuccinic acid, sulfomethylsuccinic acid,sulfofumaric acid, sulfomaleic acid, 2-sulfobenzoic acid, 3-sulfobenzoicacid, 4-sulfobenzoic acid, 5-sulfosalicylic acid, 4-sulfophthalic acid,5-sulfoisophthalic acid, 2-sulfoterephthalic acid andnaphthalenesulfonic acid.

Among these organic sulfonic acids, methanesulfonic acid,2-hydroxyethanesulfonic acid, 2-hydroxypropane-1-sulfonic acid,trichloromethanesulfonic acid, trifluoromethanesulfonic acid,benzenesulfonic acid, toluenesulfonic acid, phenolsulfonic acid,cresolsulfonic acid, anisolesulfonic acid and naphthalenesulfonic acidare used more preferably, and methanesulfonic acid, toluenesulfonic acidand phenolsulfonic acid are particularly preferred.

As the organic carboxylic acid used as the counter anions, for example,monocarboxylic acids such as formic acid, acetic acid, propionic acid,butanoic acid and octanoic acid; dicarboxylic acids such as oxalic acid,malonic acid and succinic acid; hydroxycarboxylic acids such as lacticacid, glycolic acid, tartaric acid and citric acid; andhalogen-substituted carboxylic acids such as monochloroacetic acid,dichloroacetic acid, trichloroacetic acid, trifluoroacetic acid andperfluoropropionic acid are preferably used.

Among these organic carboxylic acids, formic acid, acetic acid, oxalicacid, lactic acid, trichloroacetic acid, trifluoroacetic acid andperfluoropropionic acid are preferred, and acetic acid, lactic acid andtrifluoroacetic acid are particularly preferred.

In the present invention, complexes of silver ion and/or copper ion andphosphines, nitrogenated heterocyclic compound, or a compound havingthiol, thioether or disulfide bond can also be used independently, ormay be used by mixing two or more types.

As for complexing agent for forming the aforesaid complex with silver orcopper, only complexing agent can further be added besides the aforesaidcomplex, in order to further stabilize flux.

The flux of the present invention can be manufactured by mixing therespective compositions as above described, and then melting whileheating. The flux of the present invention may contain othercompositions such as thixotropy imparting agent, in addition to theabove-mentioned compositions. As thixotropy imparting agent, there are,for example, hydrogenated castor oil (hardened castor oil), beeswax,camauba wax, amide stearate, and hydroxy ethylene stearate bisamide. Thecontent of thixotropy imparting agent is preferably in an amount of 1.0to 25% by weight of the total amount of flux.

Additionally, the flux of the present invention may be used togetherwith synthetic resin such as polyester resin, phenoxy resin, and terpeneresin, which have heretofore been known and used as base resin of flux.It is also able to further add various additives such as antioxidant,fungicide, and delustering agent.

The flux of the present invention is particularly suitably used in caseswhere electroless nickel plating to the lands of a substrate isconducted in advance, or gold plating to the electroless nickel platingis also conducted in advance. While no limitations are imposed on themetal of the lands, to which electroless nickel plating is conducted,copper is preferred.

Although no limitations are also imposed on the type of solder alloyemployed in soldering, general tin/lead alloy solder is usable.Alternatively, there may be used so-called lead-free solder in whichmetal such as silver, zinc, bismuth, indium, or antimony is mixed withtin that is used as base.

In accordance with the first soldering method of the present invention,first, the above-mentioned flux is printed by screen printing etc. orapplied on a substrate having lands which have on their surfaceselectroless nickel plating or further have gold plating on theelectroless nickel plating, and then heating to form a barrier layer ofsilver and/or copper on the surfaces of the lands. Heating temperatureis preferably 150° C. to 280° C.

Subsequently, solder balls are placed on the lands and then heated tohave the solder balls reflow so as to connect the solder balls to thelands. From an environmental standpoint, the solder balls are preferablyformed of lead-free solder as described above. The reflow of solderballs is done by preheating at, for example, 150 to 200° C., and thenheating at 170 to 280° C., or alternatively, by directly heating withoutpreheating. Printing and reflow may be carried out in the atmosphere ofair, or in the atmosphere of an inert gas such as nitrogen, argon, andhelium.

In accordance with the second soldering method, first, theabove-mentioned flux is printed or applied on a substrate having copperlands which have on their surfaces electroless nickel plating or furtherhave gold plating on the electroless nickel plating. Subsequently,solder balls are placed on the lands and then heated to have the solderballs reflow so as to form a barrier layer of silver and/or copper onthe surfaces of the lands, and connect the solder balls via the barrierlayer to the lands. The conditions of reflow are based on the firstsoldering method.

In accordance with the third soldering method, paste that is a mixtureof the above-mentioned flux and solder powder is printed on a substratehaving lands which have on their surfaces electroless nickel plating orfurther have gold plating on the electroless nickel plating, and thenheated to have the paste reflow so as to form solder alloy on the lands.The reflow temperature of the solder balls is preferably 150° C. to 280°C. This enables to form solder alloy on the lands, via a barrier layerof silver and/or copper, on the surfaces of the lands.

In either case, the barrier layer to be formed is composed of a metalliclayer of silver and/or copper, which is substituted by nickel in theaforesaid electroless nickel plating and deposited. The flux may beprinted overall on the substrate surface.

Alternatively, in the present invention, paste that is a mixture of theabove-mentioned flux for soldering and solder powder may be printed on asubstrate having copper lands which have on their surfaces electrolessnickel plating or further have gold plating on the electroless nickelplating, and then heated to have the paste reflow so as to form solderalloy on the lands.

Examples of the present invention will be described below. It isunderstood, however, that the examples are for the purpose ofillustration and the invention is not to be regarded as limited to anyof the specific materials or condition therein.

EXAMPLES

[Silver or Copper Compound]

As silver or copper compounds, the following materials were selected.

-   A: [Ag{P(C₆H₅)₃}₄]⁺CH₃SO₃ ⁻-   B: [Ag]⁺C₆H₄N₃ ⁻-   C: [Ag]⁺C₂H₂N₃S⁻-   D: [Cu{P(C₆H₅)₃}₃]⁺CH₃SO₃ ⁻-   E: [Cu]⁺C₆H₄N₃ ⁻-   F: [Cu]⁺C₂H₂N₃S⁻

Comparative Examples

-   G: C₇H₁₅COOAg-   H: (C₇H₁₅COO)₂Cu    [Preparation of Silver or Copper Compound Mixed Flux]

With the use of a triple-roll, the previously selected silver or coppercompound and base flux (RMA flux) were mixed uniformly to prepare silveror copper compound mixed flux. The composition of the used base fluxwere as follows: WW class tall rosin 70 parts by weight Hexyl carbitol25 parts by weight Hydrogenated castor oil 4.8 parts by weight Isopropylamine HCl 0.2 parts by weight

The mixing ratios of the type of silver or copper compounds and RMA fluxare indicated in Table 1. The mixing amounts of the respectivecompositions are expressed in % by weight. TABLE 1 RMA Flux Silver orCopper Compound Flux 1 100 — Flux 2 90 A: 10 Flux 3 90 B: 10 Flux 4 90C: 10 Flux 5 90 D: 10 Flux 6 90 E: 10 Flux 7 90 F: 10 Flux 8 90 G: 10Flux 9 90 H: 10

Experimental Example 1

The respective fluxes in Table 1 were printed overall and in a thicknessof 100 μm on a tandem-pattern substrate, and heated by using reflowprofile whose maximum temperature was 250° C. The substrate was thenimmersed in an ultrasonic washer filled with butyl carbitol solution of60° C., to remove the flux. Then, insulating resistance was measuredafter leaving it for 168 hours in an atmosphere having a temperature of85° C. and a relative humidity of 85%, while applying a voltage of DC50Vto the substrate (Based on the test method of JIS Z3197).

Experimental Example 2

Lands were formed on a substrate, which were 0.4 mm in diameter and hadnickel electroless plating to copper and further had gold flash platingthereon, and the respective fluxes in Table 1 were printed overall andin a thickness of 100 μm on the substrate. Then, solder balls of tin-37lead having a diameter of 0.4 mm were placed on the lands, and allowedto reflow at 220° C. so as to connect the solder balls to the lands.Subsequently, the substrate was immersed in an ultrasonic washer filledwith butyl carbitol solution of 60° C., to remove the flux.

Thereafter, with regard to resist portions in the vicinity of enelectrode pattern, elemental analysis with an energy dispersive X-rayfluorescence analyzer (EDX) was conducted to examine the presence orabsence of the reduction deposition of the metal of silver or copper.

In addition, with regard to the soldered portions of the solderedsubstrate, tension test was conducted to measure bonding strength(DAGE-SERIES-4000P, manufactured by DAGE Incorporation was used). Themeasurement was made 30 times per flux, and the average was used asbonding strength, and the minimum value was used as minimum bondingstrength.

[Test Results]

The results of the foregoing tests are shown in Table 2. TABLE 2 Example1 Example 2 Insulating Reduction Bonding Minimum Bonding Flux ResistanceDeposition Strength (N) Strength 1 1.3 × 10¹² Ω nothing 12.6 5.5 2 1.8 ×10¹² Ω nothing 15.3 12.3 3 1.5 × 10¹¹ Ω nothing 14.5 12.8 4 1.7 × 10¹¹ Ωnothing 14.8 12.6 5 2.3 × 10¹² Ω nothing 16.1 13.8 6 2.7 × 10¹² Ωnothing 15.1 12.2 7 1.9 × 10¹² Ω nothing 15.8 13.3 8   <1 × 10⁶ Ωpresence 14.5 11.8 9   <1 × 10⁶ Ω presence 14.1 11.5

As apparent from Table 2, in the cases of using Flux 2 to Flux 7, nometal was detected at the resist portions other than the electrodepattern, and no drop in insulating resistance was observed. Also, as forbonding strength, the average bonding strength increases, and such anextremely low bonding strength as in the case of connecting with Flux 1was not observed, resulting in small variations of bonding strength.Whereas in Flux 8 and Flux 9, which used octanoic acid silver andoctanoic acid copper, silver or copper caused substitution deposition atthe nickel portion of the lands, and metallic silver or metallic copperwas deposited due to reduction deposition, and this was detected, asforeign matter, over the entire surface of an application portion, sothat insulating resistance was lowered remarkably.

Also in the case of individually printing per pad, from the fact thatsilver or copper was detected in the vicinity of the pad due to heatingdroplet during reflow, it is clear that the insulating propertiesbetween the electrode patterns will be impaired.

1. A flux containing resin having film forming ability, activator,solvent, and at least one complex selected from silver complex andcopper complex, said flux being used when soldering is performed onto acircuit having electroless nickel plating or further having gold platingon said electroless nickel plating.
 2. The flux according to claim 1wherein said complex does not substantially cause substitutiondeposition of at least one selected from silver and copper on a circuitpattern at temperatures of less than 150° C., and causes substitutiondeposition at temperatures of not less than 150° C.
 3. The fluxaccording to claim 2 wherein said complex does not substantially causedeposition of at least one selected from silver and copper at portionsother than a circuit pattern at temperatures of not more than 280° C. 4.The flux according to claim 1 wherein said complex is a complex ofsilver ion and/or copper ion and phosphine, nitrogenated heterocycliccompound, or a compound having thiol, thioether or disulfide bond. 5.The flux according to claim 4, wherein said phosphine is aryl phosphineor alkyl phosphine represented by the following general formula
 1.

wherein R₁, R₂ and R₃ each represents a substituted or non-substitutedaryl group, or a substituted or non-substituted chain or cyclic alkylgroup having 1 to 8 carbon atoms; hydrogen of the aryl group may besubstituted with an alkyl having 1 to 8 carbon atoms, an alkoxy grouphaving 1 to 8 carbon atoms, a hydroxyl group, an amino group or ahalogen atom at any position; hydrogen of the alkyl group may besubstituted with an alkoxy group having 1 to 8 carbon atoms, an arylgroup, a hydroxyl group, an amino group or a halogen at any position;and R₁, R₂ and R₃ may be the same or different.
 6. The flux according toclaim 5, wherein said aryl phosphine or the alkyl phosphines aretriphenyl phosphine, tri(p-tolyl)phosphine,tri(p-methoxyphenyl)phosphine, trioctyl phosphine andtris(3-hydroxypropyl)phosphine.
 7. The flux according to claim 4,wherein said nitrogenated heterocyclic compounds is at least oneselected from five-ring compound, six-ring compound and derivativesthereof.
 8. The flux according to claim 4, wherein said complex containorganic sulfonic acid ion, organic carboxylic acid ion, halogen ion,nitric acid ion or sulfuric acid ion as counter anion.
 9. The fluxaccording to claim 8, wherein said organic sulfonic acid is at least oneselected from organic sulfonic acids represented by the followinggeneral formulas 2, 3 and 4.(X₁)_(n)—R₄—SO₃H   general formula 2: wherein R₄ represents an alkylgroup having 1 to 18 carbon atoms, an alkenyl group having 2 to 18carbon atoms or an alkynyl group having 2 to 18 carbon atoms, X₁represents a hydrogen, a hydroxyl group, an alkyl group having 1 to 8carbon atoms or an alkoxy group, an aryl group, an aralkyl group, acarboxyl group or a sulfo group, n represents an integer of 0 to 3, andX₁ may be bonded at any position of R₄.(X₂)_(n)—R₅—(SO₃H)_(m)   general formula 3: wherein R₅ represents analkyl group having 1 to 18 carbon atoms or an alkylene group having 1 to3 carbon atoms, and when an alkylene group, a hydroxyl group may bebonded at any position of the alkylene group; X₂ represents chlorineand/or fluorine; n represents an integer of not less than 1 and not morethan the number of hydrogen capable of bonding with R₅; and m representsan integer of I to
 3. X₂ may be bonded with R₅ at any position.

wherein X₃ represents a hydroxyl group, an alkyl group having 1 to 18carbon atoms, an alkoxy group having 1 to 8 carbon atoms, an aryl group,an aralkyl group, an aldehyde group, a carboxyl group, a nitro group, amercapto group, a sulfo group or an amino group, or two adjacent X₃(s)may form a ring to form a naphthalene ring with a benzene ring, and nrepresents an integer of 0 to
 3. 10. The flux according to claim 8,wherein said organic carboxylic acid ion is at least one selected fromformic acid, acetic acid, oxalic acid, lactic acid, trichloroaceticacid, trifluoroacetic acid and perfluoropropionic acid.
 11. A circuitboard comprising: lands having on their surfaces electroless nickelplating or further having gold plating on said electroless nickelplating; and a barrier layer formed on the surfaces of said lands, saidbarrier layer being composed of a metallic layer of at least oneselected from silver and copper that are substituted by nickel in saidelectroless nickel plating and deposited, and said metallic layer beingformed by printing or applying a flux for soldering according to claim 1onto the surface of said circuit board, followed by heating.
 12. Acircuit board comprising: lands having on their surfaces electrolessnickel plating or further having gold plating on said electroless nickelplating; and a barrier layer formed on the surfaces of said lands, saidbarrier layer being composed of a metallic layer of at least oneselected from silver and copper that are substituted by nickel in saidelectroless nickel plating and deposited, and said metallic layer beingformed by heating involved when a flux for soldering according to claim1 is printed or applied onto the surface of a circuit board, and thensolder balls placed on said lands are allowed to reflow so as to connectsaid solder balls to said lands.
 13. A soldering method comprising thesteps of: printing or applying a flux for soldering according to claim 1onto a substrate having copper lands which have on their surfaceselectroless nickel plating or further have gold plating on saidelectroless nickel plating, and heating to form a barrier layer of atleast one selected from silver and copper on the surfaces of said lands;and placing solder balls on said lands with said barrier layer formed ontheir surfaces, and heating to have said solder balls reflow so as toconnect said solder balls to said lands.
 14. A soldering methodcomprising the steps of: printing or applying a flux for solderingaccording to claim 1 onto a substrate having copper lands which have ontheir surfaces electroless nickel plating or further have gold platingon said electroless nickel plating; and placing solder balls on saidlands, and heating to have said solder balls reflow so as to form on thesurfaces of said lands a barrier layer of at least one selected fromsilver and copper, and connect said solder balls via said barrier layerto said lands.
 15. The soldering method according to claim 13 or 14,wherein said solder balls are lead-free.
 16. A soldering methodincluding: printing paste, which a mixture of a flux for solderingaccording to claim 1 and solder powder, on a substrate having lands thathave on their surfaces electroless nickel plating or further have goldplating on said electroless nickel plating, and then heating to havesaid paste reflow so as to form solder alloy on said lands.
 17. Asoldering method according to claim 13, 14, wherein the reflowtemperature of solder balls is 150° C. to 280° C.
 18. A soldering methodaccording to claim 15, wherein the reflow temperature of solder balls is150° C. to 280° C.